The present application is based on and claims priorities from Japanese Patent Application Nos. Hei. 6-230189 and Hei. 6-230190 both filed Sep. 1, 1994, the disclosures of which are incorporated by reference herein.
The present invention relates to a magneto-optical disk apparatus and, more specifically, to a magneto-optical disk apparatus having an external magnetic field generating device for applying magnetic fields to a magneto-optical disk in different directions in recording and erasing data onto and from it.
It is known that in a magneto-optical disk apparatus, data recording and erasure are effected by changing the direction of magnetization in a magneto-optical disk by heating a portion thereof with laser light or the like while applying a magnetic field to it. For example, as shown FIG. 1A, in when a magneto-optical disk DISK is uniformly magnetized downward, if a certain portion of the magneto-optical disk DISK is heated to a temperature higher than a prescribed temperature (Curie point) by causing an optical pickup 101 to illuminate that portion with laser light L while an upward external magnetic field H.sub.X is generated by an external magnetic field generating device 100, the direction of magnetization at the heated portion is changed to the upward direction due to the external magnetic field H.sub.X. As a result, the magnetization direction at this portion is made different from that at the other portions. In this manner, data can be recorded in the form of magnetizations having respective directions.
To erase recorded data, as shown in FIG. 1B, a recorded portion is heated by illumination with laser light L in the same manner as in the above while a downward external magnetic field H.sub.X is applied to the magneto-optical disk DISK by the external magnetic field generating device 100. As a result, the magnetization direction at the heated portion is changed to the downward direction, i.e., equal to that at the other portions, which means data erasure.
Since, as described above, the magneto-optical disk apparatus needs external magnetic fields in different directions for data recording and erasure, various conventional devices for reversing the external magnetic field have been proposed. For example, Japanese Examined Patent Publication No. Hei. 5-60162 discloses a configuration schematically shown in FIG. 2. A permanent magnet 200 for applying an external magnetic field to a magneto-optical disk DISK is so supported as to be rotatable about a shaft in an eccentric manner. The permanent magnet 200 is rotated by 180.degree. by reversing the direction of a current flowing through a coil 201 that surrounds the permanent magnet 200, in which a repulsive force due to magnetic fields of the permanent magnet 200 and the coil 201 is utilized. As a result, the direction of the external magnetic field H.sub.X applied to the magneto-optical disk DISK by the permanent magnet 200 is reversed. In this manner, data is recorded and erased by means of external magnetic fields in the respective directions. Reference numeral 202 denotes an optical pickup device.
However, the technique proposed in the above publication involves a physical operation of rotating the permanent magnet about the shaft when the direction of the external magnetic field is reversed. In particular, in the above example, it is necessary to rotate the permanent magnet by 180.degree. (half rotation). The rotation takes time, during which neither data recording nor erasure can be performed on the magneto-optical disk. That is, being restricted by the physical operation time of the permanent magnet, the access time of data recording and erasure cannot be shortened.
From the viewpoint of access time, a configuration would be advantageous in which the external magnetic field generating device is implemented by a coil and the direction of the external magnetic field is reversed by switching the direction of a current flowing though the coil. However, this configuration consumes more electric power than the configuration using the permanent magnet, because a relatively large current needs to flow through the coil.
Further, in the technique proposed in the above publication, the orientation of the poles of the permanent magnet for generating the external magnetic field is set by the magnetic force acting between the permanent magnet and the coil. Therefore, it is necessary to energize the coil when rotating the magnetic field by 180.degree. to reverse the direction of the external magnetic field, and when holding the permanent magnet with its poles oriented in either way to apply an upward or downward external magnetic field to the magneto-optical disk. That is, the coil needs to be energized all the time. Therefore, not only is power consumption is large but also heat generation from the coil is not negligible, which will make the magneto-optical disk apparatus more complex or increase its size due to, for instance, the necessity of adding a heat dissipation structure.